Magnetic recording media require a high level of various characteristics and many improvements have been made. One of the required characteristics is S/N. To improve the S/N of coating type magnetic tapes, it is required that ferromagnetic substances are uniformly dispersed in binders and the surfaces of coated magnetic layers are made smooth. From this point of view, it is prescribed in JP-A-60-187931 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") that the water content of the ferromagnetic substances is adjusted to 0.8 wt % or higher by moisture conditioning.
Dispersibility and orientation are improved in the above technique, because the water contents of the ferromagnetic substances are not lower than 0.8 wt %. However, water on the surfaces of the ferromagnetic substances reacts with isocyanate compounds in coating solutions, whereby the pot life of the coating solutions is shortened and the surface profile of the resulting coated magnetic tapes is deteriorated. Further, the reaction of the isocyanate compounds is hastened by water and the curing of the coating film is also hastened. Therefore, calender moldability is deteriorated in the production thereof, the surface properties of magnetic tapes are deteriorated and a satisfactory S/N cannot always be obtained.
Recently, high-density recording has been required for magnetic recording media and studies in highly filling the ferromagnetic substances have been made. Investigation of highly filling the ferromagnetic substances can be narrowed down to two points, the pulverization of the ferromagnetic substances and the improvement of the fillability thereof (lower to higher fill of the substances).
In order to improve recording density, high filling and the smoothing of magnetic layers are made and an effort to minimize spacing loss must be made.
Various means for high filling have been studied. As a means thereof, there is known a method wherein a powder compacting treatment (or compacting treatment) is carried out during the course of the preparation of the ferromagnetic substances themselves or a kneading treatment is carried out during the course of the preparation of the coating solutions of the ferromagnetic substances.
The term "powder compacting treatment" (or "compacting treatment") as used herein refers to a crushing (pulverizing) treatment of magnetic powders, which is carried out by using a ball mill, a colloid mill or the like. Namely, it is a means for improving fillability by removing air contained in magnetic powder to lower the bulk of the magnetic powder (namely to increase bulk density).
The term "kneading treatment" as used herein refers to a treatment in which kneading is carried out by using a roll mill, a kneader or the like before the coating solution of magnetic substance is subjected to a dispersion treatment in a dispersion device such as a sand grinder. Namely, it is a means for improving fillability.
It was known for a long time that the compacting treatment or the kneading treatment is effective in improving high filling (namely in improving audio characteristics). For example, compacting kneading treatment is described in Material, Vol. 15, No. 150, pages 160 to 165 (1966) and kneading treatment is described in JP-A-59-165237, JP-B-57-42888 (the term "JP-B" as used herein means an "examined Japanese patent publication"), etc.
High filling for high-density recording could be achieved by conventional compacting treatment or kneading treatment. However, the magnetic characteristics of such magnetic recording media are not always improved. There is much difficulty in improving video characteristics (particularly S/N) as well as audio characteristics in the case of video tapes having a multilayer structure in particular.
For example, when the compacting treatment is intensively carried out, magnetic substance is finely divided. However, acicular magnetic substance is broken and acicular ratio is lowered. As a result, squareness ratio and coercive force are lowered and video characteristics (particularly S/N) are deteriorated when a magnetic tape is prepared.
When the kneading treatment is intensively carried out, fillability can be improved. However, when the kneading treatment is conducted to excess, the acicular ratio of the acicular magnetic substance is deteriorated, coercive force is reduced and squareness ratio is lowered. Hence, video characteristics are damaged.
Accordingly, it has been highly desirable to develop a magnetic recording medium having excellent magnetic characteristics, particularly a magnetic recording medium having a multilayer structure, which is excellent in video characteristics as well as audio characteristics even when there are used magnetic substances which have been subjected to the compacting treatment or the kneading treatment for the purpose of high filling.